Sunday was the last full day of campaigning ahead of the vitally important Iowa caucuses – the first real test for 15 Republican and Democratic candidates hoping to become the next president of the United States.

The caucuses are the first event of the presidential election in which voters' choices actually count. It also is the time when the large field of candidates could be pared down by a poor showing.

The latest polls going into Monday's caucuses have billionaire Donald Trump leading the Republican hopefuls with 28 percent support, followed by Senator Ted Cruz with 23 percent.

Appearing on ABC television's This Week broadcast Sunday, the straight talking and caustic Trump again tore into Cruz, calling him a liar whom no one in the Senate wants in the White House. He said only a Trump presidency can work with all sides.

"You need a deal maker, too. You can't have just somebody standing in the Senate floor and nobody even endorses you. Not one endorsement of Cruz because he's a nasty guy. Nobody likes him.... you can't run a country that way...it will be a total mess," Trump said.

WATCH: VOA Snapshot of the Iowa Race - Republicans

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On NBC's Meet the Press, Cruz said he is the only true conservative who can win and refused to trade insults with other candidates.

"As others attack me, I don't respond in kind. I don't engage…. I'll sing Donald's praises. I like Donald. I think he's bold and brash.... I think his policies are liberal. I think he's been too willing to cut a deal to get along with Democrats and grow government and support cronyism," Cruz said.

Democratic presidential candidate Hillary Clinton speaks at a rally at Abraham Lincoln High School in Council Bluffs, Iowa, Jan. 31, 2016.

For the Democrats, the polls have former secretary of state Hillary Clinton and Vermont Senator Bernie Sanders just about tied, with Clinton holding a slight lead.

Clinton told ABC Sunday that the controversies she has faced over the years, including the latest release of State Department e-mails, will not hurt her in the general election.

"I've been subjected, as you know so well, to years of scrutiny and I'm still standing, talking to you in the lead here in Iowa for the caucuses... and it's a very tough gauntlet to run, and it there are issues - the Republicans and their allies on the right believe they could use that to bring a Democrat down…. I feel vetted, I feel ready... I feel strong," said Clinton.

Sanders told the same broadcast that turnout is the key to his success Monday.

"If working people and lower income people and young people come out to vote in significant numbers tomorrow (Monday) night, we're going to win this thing and pull off one of the great political upsets in recent history," he said.

WATCH: VOA Snapshot of the Iowa Race - Democrats

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